I. Field of the Invention
The present invention relates generally to wireless communications, and more particularly to correction of communications signals traversing between transmitting and receiving stations that are moving relative to each other, to compensate for Doppler effects.
II. Related Art
Today, wireless communications systems are used for a variety of purposes, including local and global telephonic communications, television broadcasts, and terrestrial positioning, just to name a few. One component of all of these systems is the relationship between a transmitting station and a receiving station and, more particularly, the relative velocity between the two stations. One example of this is in the field of ground to satellite communications. Depending upon the particular system, satellites might communicate with a variety of terrestrial stations, from fixed ground stations that are designed to handle a high volume of traffic, to wireless telephones carried by an individual user. Satellites may also communicate with other satellites moving in different orbital planes and/or in different directions. Other examples may include communications with and between high speed aircraft or even high speed trains, of the type found, for example, in Europe and Japan.
Designers of such communications systems must often compensate for Doppler effects where the transmitting station is moving relative to the receiving station(s) with which it is communicating. The Doppler effect was discovered by Christian Johann Doppler who first stated the principle in 1842. The Doppler effect is the apparent variation in the frequency of an emitted wave, as the source of the wave moves toward or away from the observer. Only the radial (approaching or receding) component of motion produces this phenomenon. If the source of a wave is approaching an observer, the apparent frequency increases and the apparent wavelength decreases. If the source is receding from an observer, the apparent frequency decreases and the apparent wavelength increases. If there are several observers, each moving radially at different speeds relative to the source of an EM field, every observer will perceive a unique frequency and wavelength for the EM field produced by the source.
The frequency perceived by an observer is determined as follows. Let the speed of propagation of an electromagnetic (EM) field, in meters per second (m/s), be represented by c, and the (radial) speed component of the observer (for example, the satellite) relative to the source (for example, the terrestrial transmitter), also in meters per second, be represented by v. Further, let the apparent (observed) frequency of the EM wave, in hertz (Hz), be represented by fapp, and the actual frequency, also in Hz, by f. Then:fapp=f(1±v/c) In free space, the value of c is approximately 300,000,000 m/s. If the transmitter and receiver are moving (relatively) toward each other, the relative separation is decreasing and the velocity component of the equation is negative. Conversely, if the transmitter and receiver are moving (relatively) away from each other, the relative separation is increasing and the velocity component of the equation is positive.
The above formula is reasonably valid for velocities up to about 10 percent of the speed of light. For greater speeds, relativistic time dilation occurs, reducing the frequency and increasing the wavelength independently of Doppler effect.
The Doppler effect is significant in applications where the product of velocity and frequency is high enough so that bandwidth will be significantly affected. Such is the case with low-earth-orbit (LEO) satellite systems, where the frequency ranges are on the order of 1-2 GHz for forward link signals (that is, signals transmitted from a base station to a satellite) and on the order of 5-6 GHz for reverse link signals (that is, signals transmitted from a satellite to a base station). LEO satellites typically are constantly moving relative to each other and to points on the earth's surface. This causes variations in the frequencies and wavelengths of received signals. In geostationary satellite systems, Doppler effect is not a factor unless the end user (mobile transceiver) is moving a a high speed such as when on board a high-speed train or high-speed aircraft.
The Doppler effect can have a variety of effects on satellite communications, depending in part upon the types of signals used within the system. For example, the Doppler effect will cause an apparent shift in the carrier frequency for those ground-to-satellite communications signals employing a carrier. This effect is referred to as “frequency Doppler.” For those signals that also employ a spreading code, such as code division multiple access (CDMA) signals, the Doppler effect will also cause an apparent shift in the period of the spreading code. This effect is referred to as “code Doppler.” Though frequency Doppler and code Doppler are two manifestations of the same Doppler effect, the implications are quite different in terms of their impact on CDMA based satellite communication systems.
A need, therefore, exists for an improved apparatus that compensates for Doppler effects within a wireless communications system. This need is especially acute in satellite communications systems.